Decorative laminating paper forms part of a decorative, thermosetting coating material, used with preference for finishing furniture surfaces and for laminate flooring. Laminates is the term used to denote materials in which, for example, several impregnated, stacked layers of paper, or paper and hardboard or fibreboard, are pressed together. The use of special synthetic resins achieves extraordinarily high resistance of the laminates to marring, impact, chemicals and heat.
The use of special-purpose papers (decorative laminating papers) permits the production of decorative surfaces, where the decorative laminating paper serves not only as facing paper, e.g. to hide unattractive wood material surfaces, but also as a carrier for the synthetic resin.
The demands imposed on decorative laminating paper include, among others, opacity (hiding power), light-fastness (greying resistance), colour-fastness, wet strength, suitability for impregnation and printability.
In principle, a pigment based on titanium dioxide is eminently suitable for achieving the necessary opacity of the decorative laminating paper. During paper manufacture, a titanium dioxide pigment, or a titanium dioxide pigment suspension, is usually mixed with a pulp suspension. In addition to pigment and pulp as the feedstock, use is generally also made of auxiliaries, such as wet-strength agents, and further additives, such as certain fillers, where appropriate. The interactions of the individual components (pulp, pigment, auxiliaries and additives, water) with each other contribute to formation of the paper and determine the retention of the pigment. Retention is the capacity for retaining all inorganic substances in the paper during production.
A number of titanium dioxide pigments exist for use in decorative laminating paper. Their key properties include not only good brightness and opacity, but also light-fastness.
Titanium is generally known to be photochemically active. A decorative laminating paper pigmented with titanium dioxide displays increasing greying when exposed to UV radiation in the presence of moisture and oxygen. Light-fastness is primarily taken to mean the resistance of laminates to greying when exposed to UV radiation.
To improve the light-fastness (greying resistance) of decorative laminating paper, the titanium dioxide pigment is customarily coated with aluminium compounds, particularly with aluminium phosphate.
For example, U.S. Pat. No. 5,114,486 discloses coating with zinc/aluminium phosphate in order to improve greying resistance.
U.S. Pat. No. 5,785,748 describes a method for uniform coating of titanium dioxide with aluminium phosphate, where a mixture of concentrated phosphoric acid and an aluminium compound is added to a titanium dioxide suspension, and aluminium phosphate is precipitated at a pH value of 3.5 or more.
WO 2004/061013 A2 discloses a titanium dioxide pigment with good greying resistance for use in decorative laminating paper that is provided with an aluminium phosphate coating and demonstrates particularly favourable surface properties as regards the isoelectric point and the zeta potential. The aluminium phosphate layer is precipitated at a constantly maintained pH value of 7.
In an advanced development of this method according to DE 10 2006 045 244 A1, the coated pigment is finally subjected to heat treatment.
According to DE 103 32 650 A1, it is possible to manufacture a titanium dioxide pigment with high greying resistance that simultaneously displays improved retention and opacity. The method is characterised in that an aluminium component and a phosphorus component are added to a titanium dioxide suspension at a constantly maintained pH value of at least 10, after which the pH value is reduced to below 9 in order to precipitate aluminium phosphate.